Replace jquery-datetimepicker with native date input (#11684)

This removes the jQuery plugin as well as the associated config options.
Native input[type=date] does not require a language attribute as it is
localized by default, except for the placeholder attribute for which I
currently piggy-back the repo.issues.due_date_form localization option.

Implementation should pretty much match GH. Of note is that Safari does
not provide a UI for this input type, but I don't think providing one is
neccessary and GH did not bother either.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

webpack.config.js

@@ -101,10 +101,6 @@ module.exports = {
         exclude: /node_modules/,
         loader: 'vue-loader',
       },
-      {
-        test: require.resolve('jquery-datetimepicker'),
-        use: 'imports-loader?define=>false,exports=>false',
-      },
       {
         test: /\.worker\.js$/,
         exclude: /monaco/,